No description available.

Since fires can develop from endogenous smouldering combustion processes deep inside packed beds, especially domestic refuse beds, it is a major task in early fire detection to detect the indications of such combustions as early as possible. Since it is believed that the surface temperature distribution of the bed is affected by the heat and mass transfer from a source of endogenous combustion deep within the bed, the measurement of the surface temperature using IR-Thermography (IRT) has been supposed to be the most promising technique in early combustion detection. The present work thus deals with the heat and mass transfer from endogenous sources of combustion in packed beds, particularly domestic refuse beds, in order to predict the temperature distribution inside and at the surface of these beds and thus, to allow for an assessment of IRT based early combustion detection systems. An IR-thermographically measurable surface temperature increase will only be achieved by sufficient heat transfer from the source of combustion. Experimental procedures and mathematical modelling have shown that the heat transfer by conduction and radiation is ineffective and therefore, no indication will be obtainable from the surface temperature distribution. A more satisfactory increase in the surface temperature is given as soon as additional heat is transferred by the diffusion of gaseous combustion products. As a result, the heat transfer by convection from the hot combustion gases is theoretically analysed, in particular the way in which the gases flow from the combustion to the surface of the bed. The results obtained show that the gas flow is initiated and maintained by buoyancy and thus the gas tends to flow vertically towards the surface with minimal collateral diffusion. It was also shown that heterogeneous polydispersed beds can be treated as homogeneous monodispersed beds as long as average values for the characteristic bed properties can be obtained. Based upon that, a mathematical continuum model was derived by which the temperature distribution inside and at the surface of the bed could be predicted. The theoretically obtained results and their implications were then experimentally verified, confirming that heat is predominantly vertically transferred by the gaseous combustion products. The two final sets of experiments were undertaken in a 27 m3 batch of a representative sample of domestic refuse and a batch of wood chips. Comparing the experimental results with the mathematical predictions, a certain deviation becomes apparent, which is attributed to the not exactly one-dimensional condition inside the bed and especially to the effect of condensation and re-evaporation of the water content of the combustion gas, which has not been included in the model. The temperature of hot spots at the surface, that have the size of only a few square-centimetres, increased to the dew point temperature of the combustion gas, i.e. 65 °C to 85 °C, within the first hour after ignition but remained almost constant at this level for several hours. About 30 minutes before the combustion proceeded to the surface, their temperature increased rapidly but not their size. The rapid temperature increase was attributed to the condensation and re-evaporation ceasing because the entire bed had heated up to a temperature at which these effects no longer occur. All results obtained, especially for the surface temperature development, allow for an assessment of IRT based early combustion detection systems. Whilst, on one hand, sources of endogenous combustion are principally detectable from the surface temperature distribution, on the other hand, the reliable detection of the very small hot spots requires a spatial resolution of the system, which is up to ten times higher than recent state-of-the-art systems can provide.

No description available.

This study compared experimental and Computational Fluid Dynamics (CFD) results for heating and cooling in a packed bed (N=5.33). The experimental data was compared between heating and cooling, and was also used to validate the CFD model. The validated models were used to compare theoretical heat transfer parameters. For the experiments, it was found that the effective thermal conductivity was comparable for heating and cooling, and the wall Nusselt number for heating was higher. For the CFD results, it was found that both the wall Nusselt number and effective thermal conductivity were comparable for heating and cooling. The wall Nusselt number was slightly higher for cooling, however this difference decreased as the Reynolds number increased.

This study compared experimental and Computational Fluid Dynamics (CFD) results for heating and cooling in a packed bed (N=5.33). The experimental data was compared between heating and cooling, and was also used to validate the CFD model. The validated models were used to compare theoretical heat transfer parameters. For the experiments, it was found that the effective thermal conductivity was comparable for heating and cooling, and the wall Nusselt number for heating was higher. For the CFD results, it was found that both the wall Nusselt number and effective thermal conductivity were comparable for heating and cooling. The wall Nusselt number was slightly higher for cooling, however this difference decreased as the Reynolds number increased.

Packed bed reactors are widely used in the chemicals industry and have been studied carefully in the last century. Several reaction engineering models have been developed in order to predict the behavior of such reactors under specified conditions, in order to assist in the sizing during an industrial process conception. These reactors can be categorized using different parameters, and the bed-to- particle diameter ratio - N - is one of them. It has been shown that this parameter influences greatly the transfer phenomena that occur in the bed, and that for ratios under 10, particular attention is needed when considering the wall effects. An impor- tant point that has to be evaluated is the accuracy of the actual chemical reaction engineering models when simulating such beds as it is valid to question the hypoth- esis of a pseudo-continuum model when considering a low bed-to-particle diameter ratio bed. Through high precision Computational Fluid Dynamics calculations, several beds of particles are modeled and studied in term of mass dispersion and reaction rate distribution. Two reaction engineering models - a simple pseudo-continuum model with effectiveness factor, and a model we refer to as "Single pellet" model - and several correlations regarding Peclet numbers are then evaluated under the same conditions in order to determine their accuracy and reliability for that particular kind of bed. Two beds of N = 5.96 and N = 7.99 are studied for dispersion phenomena, and the bed of N = 5.96 is studied for reaction rate distribution. It is shown that the pseudo- continuum model of dispersion stands valid for the higher N, but that none of the correlations we used were able to correctly predict the behavior of the N = 5.96 bed at any of the Reynolds number we considered, only giving close behaviors. We were confronted with some difficulties regarding the reaction simulation under Fluent, but some comparisons were successfully made regarding species and reaction rate distribution in the bed.

Microwave heating is implemented in various fields such as drying, material processing, and chemical reactors. Microwaves offer several advantages over conventional heating methods: 1) microwaves deposit heat directly in the material without convection or radiation, 2) microwave heating is easy and efficient to implement, and 3) microwave processes can be controlled.In order to understand how to use microwaves more efficiently, we must understand how they affect the material with which they interact.This requires the ability to predict the temperature distribution that is achieved within the material.In recent years packed and fluidized beds have been used as chemical reactors to achieve various tasks in industry.Recent studies have shown that microwave heating offers the potential to heat the bed particles to a higher temperature than that of the fluid.This results in enhanced reaction rates and improves the overall efficiency of the reactor.T he focus of this work is to determine the temperature distributions within the packed and fluidized beds, and to determine whether the catalyst particles can be heated to a higher temperature than the gas in catalytic reactions. The beds are modeled with multiphase flow equations.The gas velocity profiles along with the solid and gas temperature profiles for packed and fluidized beds are provided. F or the fluidized beds, the hydrodynamics is modeled using FLUENT and the solid velocity profiles are also determined. / Ph. D.

When a disordered packed bed, or any heterogeneous media is studied using computational fluid dynamics, the tortuous task of generating a domain and creating a workable mesh presents a challenging issue to Engineers and Scientists. In this Thesis these challenges are addressed in the form of three studies in which both traditional and novel techniques are used to generate packed beds of spheres and cylinders for analysis using computational fluid dynamics, more specifically, the finite volume method. The first study uses a Monte-Carlo method to generate random particle locations for use with a traditional CADbased meshing approach. Computational studies are performed and compared in detail with experimental equivalent beds. In the second study, where there is a need for actual, physical beds to be studied, magnetic-resonance-imaging is used coupled with a novel approach known as image based meshing. In parallel experimental studies are performed on the experimental bed and compared with computational data. In the third study, to overcome fidelity issues with the previous approaches, a physical packed bed is manufactured which is 100% geometrically faithful to its computational counterpart to provide a direct comparison. All three computational studies have shown promising results in comparison with the experimental data described in this Thesis, with the data of Reichelt (1972) and the semi-empirical correlation of Eisfeld & Schnitzlein (2001). All experiments and computational models were carried out by the author unless otherwise stated.

Inherent safety is claimed for gas-cooled pebble bed reactors, such as the South African Pebble Bed Modular Reactor (PBMR), as a result of its design characteristics, materials used, fuel type and physics involved. Therefore, a proper understanding of the mechanisms of heat transfer, fluid flow and pressure drop through a packed bed of spheres is of utmost importance in the design of a high temperature Pebble Bed Reactor (PBR). In this study, correlations describing the effective thermal conductivity through packed pebble beds are examined. The effective thermal conductivity is a term defined as representative of the overall radial heat transfer through such a packed bed of spheres, and is a summation of various components of the overall heat transfer. This phenomenon is of importance because it forms an intricate part of the self-acting decay heat removal chain, which is directly related to the PBR safety case. In this study standard correlations generally employed by the thermal fluid design community for PBRs are investigated, giving particular attention to the applicability of the correlations when simulating the effective thermal conductivity in the near-wall region. Seven distinct components of heat transfer are examined namely: conduction through the solid, conduction through the contact area between spheres, conduction through the gas phase, radiation between solid surfaces, conduction between pebble and wall, conduction through the gas phase in the wall region, and radiation between the pebble and wall surface. The effective thermal conductivity models are typically a function of porosity in order to account for the pebble bed packing structure. However, it is demonstrated in this study that porosity alone is insufficient to quantify the porous structure in a randomly packed bed. A new Multi-sphere Unit Cell Model is therefore developed, which accounts more accurately for the porous structure, especially in the near-wall region. Conclusions on the applicability of the model are derived by comparing the simulation results with measurements obtained from various experimental test facilities. This includes the PBMRs High Temperature Test Unit (HTTU) situated on the campus of the North-West University in Potchefstroom in South Africa. The Multi-sphere Unit Cell Model proves to encapsulate the impact of the packing structure in a more fundamental way and can therefore serve as the basis for further refinement of models to simulate the effective thermal conductivity. / Thesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010

Inherent safety is claimed for gas-cooled pebble bed reactors, such as the South African Pebble Bed Modular Reactor (PBMR), as a result of its design characteristics, materials used, fuel type and physics involved. Therefore, a proper understanding of the mechanisms of heat transfer, fluid flow and pressure drop through a packed bed of spheres is of utmost importance in the design of a high temperature Pebble Bed Reactor (PBR). In this study, correlations describing the effective thermal conductivity through packed pebble beds are examined. The effective thermal conductivity is a term defined as representative of the overall radial heat transfer through such a packed bed of spheres, and is a summation of various components of the overall heat transfer. This phenomenon is of importance because it forms an intricate part of the self-acting decay heat removal chain, which is directly related to the PBR safety case. In this study standard correlations generally employed by the thermal fluid design community for PBRs are investigated, giving particular attention to the applicability of the correlations when simulating the effective thermal conductivity in the near-wall region. Seven distinct components of heat transfer are examined namely: conduction through the solid, conduction through the contact area between spheres, conduction through the gas phase, radiation between solid surfaces, conduction between pebble and wall, conduction through the gas phase in the wall region, and radiation between the pebble and wall surface. The effective thermal conductivity models are typically a function of porosity in order to account for the pebble bed packing structure. However, it is demonstrated in this study that porosity alone is insufficient to quantify the porous structure in a randomly packed bed. A new Multi-sphere Unit Cell Model is therefore developed, which accounts more accurately for the porous structure, especially in the near-wall region. Conclusions on the applicability of the model are derived by comparing the simulation results with measurements obtained from various experimental test facilities. This includes the PBMRs High Temperature Test Unit (HTTU) situated on the campus of the North-West University in Potchefstroom in South Africa. The Multi-sphere Unit Cell Model proves to encapsulate the impact of the packing structure in a more fundamental way and can therefore serve as the basis for further refinement of models to simulate the effective thermal conductivity. / Thesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010